Local multi-point distribution services (LMDS) is one way to provide high-bandwidth, interactive services as the preferred wireless platform for enhancing and extending the current global broadband communications infrastructure. LMDS is distinct from other conventional copper cable, optical fiber and low frequency wireless systems in its use of millimeter wave frequencies for wireless distribution and cellular-like layouts for spectrum reuse and spectral efficiency. The major advantages of millimeter wave distribution systems are the inherent broad transmission bandwidths that are achievable and the opportunity to minimize the use and hence, the time and cost of implementing wired infrastructure. For example, 1 GHz of bandwidth centered at 28 GHz, has been allocated by the Federal Communications Commission for a one-way television service in the New York City Metropolitan area. A system in keeping with the teachings of U.S. Pat. No. 4,747,160 to Bossard, has been deployed under this allocation. The system uses essentially omni- directional cell-sites arranged in a center-excited cellular pattern to provide one-way TV service to residential customers throughout the New York City Metropolitan area using carriers centered about 28 GHz. The signals transmitted by the cell-sites are received by high-gain/narrow-beam antenna/receiver units which are normally located just inside or outside of a subscriber's window. The received signals are then down-converted and cabled to a set-top receiver and encryption unit that processes and conveys the video and audio signals to conventional, analog televisions systems. Each cell has a channel assignment and polarization allocation that provides for the mitigation of co-channel and adjacent channel interference making possible frequency reuse, and, therefore improved spectral efficiency within a given coverage area. The center-excited coverage plan by the system disclosed in the Bossard reference is based upon geographically geographically partitioned subscribers/receivers within given cells and assumes that each receiver assigned to a cell is serviced specifically by the one cell-site transmitter and omni-directional antenna, geographically particular to the given cell in which the subscriber is located.
The system as disclosed in the reference to Bossard has shortcomings that limit the number of serviceable subscribers and hence the potential return on infrastructure investment per cell-site. To this end, the system proposed in the '160 reference, assumes essentially (a) line-of-sight propagation to most of the subscribers serviced by a given cell-site. Large area propagation shadows imposed by buildings between subscribers and their designated serving cell-sites are mitigated in the system disclosed in the '160 reference by the use of repeaters and/or roof-mounted receivers that service residences, via a wired building infrastructure. The repeaters are strategically located to enable retransmission to cover the shadowed areas. Unfortunately, a necessary consequence of the line-of-sight presumption is the large number of repeaters that are necessary in the deployment and maintenance of the system in order to provide acceptable service given the complex and changing environmental conditions that are characteristic of a large urban area. That is, the repeaters can be used in order to improve the signal quality of a shadowed subscriber, however at an increased cost. The aggressive use of repeater sites in a one-way broadcast approach, where most cell-sites transmit identical information, is not necessarily compatible with efficient frequency reuse when the information transmitted by cell-sites is not identical, or when two-way transmission systems are utilized.
Finally, atmospheric and environmentally induced fading of carriers intercepted by subscriber receivers, and in the case two-way systems by cell-site receivers impose additional limitations which can adversely affect transmission performance. In the context, fading can occur due to rain attenuation, beam deflections and multipath. The system of the '160 reference does not employ any systematic means to reduce the deleterious effects of fading due to such phenomena. Accordingly, a millimeter wave system is needed which increases the sight subscribers and provides for spectral reuse for both one way and two way communications using an affordable cell-site coverage plan.